This invention relates to navigation systems, Global Navigation Satellite Systems (GNSS), the Global Positioning System (GPS), and specifically to a method of using a two-way communications link to improve differential navigation system accuracy.
With few exceptions, GPS vertical accuracy is generally poorer than its horizontal accuracy. This is particularly acute at raised masked angles that constrict the visible sky and lead to clustering of the available satellites. A similar effect on horizontal accuracy is also found to be caused by clustering of satellites in a directional sense. For airborne applications that require vertical accuracy for guidance and control, but are saddled with operational environments of restricted satellite visibility, previous efforts have been focused primarily on lowering the range error or raising the number of ranging sources, both of which give only limited possibilities for performance improvement. Alternatively, there is much benefit to be garnered in reducing vertical dilution of precision (VDOP) itself.
Applications encountering poor VDOP will derive the most benefit. These applications include aircraft precision approach. Open sky generally yields good VDOP. However, heliports and tactical landing strips can sometimes be at locations with some restricted visibility. The flight critical nature of operation must consider satellite outages to determine service availability. Another application is automated aerial refueling where signal blockage can cause larger VDOP and HDOP (horizontal DOP) than in open sky visibility. Another application is for a MAV (miniature airborne vehicle) in urban operations where signal blockage in urban environments can have a severe impact on vertical positioning. Assisted GPS and E-911 applications in indoor environments where signal strength inside a building is very poor, so available signals are sparse and provide poor VDOP and HDOP, is another application.
A need exists for a simple, low-cost, method of time transfer to improve positioning accuracy, especially vertical, in differential navigation systems such as GPS. A robust method of time comparison that allows for non-trivial clock drift associated with low-cost frequency standards is needed.